(1) Technical Field
The technical field relates to a color image forming apparatus and in particular relates to a color image forming apparatus that uses electrophotography, such as a copier, printer, facsimile machine or the like.
(2) Description of the Prior Art
Recently, image forming apparatuses for supporting high-speed printing jobs have been developed.
For example, in the field of image forming apparatuses, processing ability of the conventional high-speed printing used to indicate a printing operation for 40 to 60 sheets per minute in terms of the number of monochrome printing with standard paper (A4 short-edge feed) a few years ago. However, at present the processing speed has been enhanced to as high as 100 to 120 sheets per minutes.
In addition, under the present circumstances, color image forming apparatuses such as full-color copiers etc., are mainly used for monochrome printing. Therefore, improvement of printing efficiency has been made by increasing the printing speed for monochrome printing and other ways.
However, as the printing speed increases, lowering of the development performance as well as toner scattering is likely to occur. Further, since the machine is frequently used for monochrome printing, extension of the maintenance interval of cleaning scattered toner and stabilization of image quality of both the full-color and monochrome printing are the matters to be considered.
As the countermeasures against the toner scatter problem there have been some proposals of image forming apparatuses, including: for example, a configuration in which blocking members for blocking air flow are laid out between each color developing device and the adjacent process unit (see Japanese Patent Application Laid-open 2003-43778 (patent literature 1); and another configuration in which the electric resistivity of the monochrome toner is specified to be equal to or greater than 1014 Ω·cm so that electricity in the toner layer will not be attracted to the opposite charge near the photoreceptor surface, whereby the toner is prevented from moving toward the electrostatic latent image (Japanese Patent Application Laid-open Hei 07-92762 (patent literature 2)).
However, provision of the blocking members between each color developing device and adjacent process unit as in patent literature 1 not only needs more parts and higher cost but also gives rise to a problem that the apparatus is unsuited for miniaturization.
Further, when the electric resistivity of the monochrome toner is specified to be equal to or greater than 1014 Ω·cm so that the toner will not be attracted to the opposite charge near the photoreceptor surface as in patent literature 2, image degradation problems such as image density lowering, etc., are prone to occur when high-speed printing is executed.